The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Polymers are used in a wide variety of ways to enhance the production of oil or gas from underground formations. Usually the function of the polymer is to control the viscosity of the aqueous fluids which are injected into the formation. For example, in water flooding the efficiency of the water flood is improved by adding a water soluble polymer to the aqueous phase and thereby decreasing the mobility difference between the injected water and the oil in place. Polymers are also used in acidizing and/or fracture acidizing in which acidic compositions are used to stimulate production of hydrocarbon from underground formations by increasing the formation porosity. A water soluble or water dispersible polymer is incorporated to increase the viscosity of the fluid so that wider fractures can be developed and live acid can be forced farther into the formations. This increases the proppant carrying capacity of the acid solutions and permits better fluid loss control.
Generally high molecular weight polymers or polymers with various gelling or crosslinking agents are used for this purpose. Most commercially available polymeric viscosifiers, however, are degraded by the hostile reservoir environment including high temperatures, acidity and extreme shear conditions, as well as by the electrolytes which are encountered in the oil recovery process. For example, hydrolyzed polyacrylamides fail in sea water solution at elevated temperatures due to precipitation of the polymer in the presence of calcium ions in the sea water. Xanthan polymers are insensitive to calcium ions but these polymers degrade at high temperatures and lose their viscosifying efficiency.
Also, conventional crosslinked polymer fracturing fluids have several inherent characteristics. The viscosity of a crosslinked polymer fluid with a given polymer concentration decreases with time and/or temperature. Hence the polymer concentration is increased in order to maintain a given or required viscosity for a longer period of time or to achieve the required viscosity at higher temperatures. The fluid loss control of the crosslinked polymer fluid in a formation with a given permeability is dependent to great extent on the polymer concentration. Increasing the polymer concentration in general will improve the fluid loss control as the polymer creates a filter cake on the face of the formation. Increasing polymer concentrations in the fluid result in lower fracture conductivity and retained permeability in the fracture faces. Both decrease the productivity of the final propped fracture. Exposure to high shear tends to degrade the properties of the crosslinked polymer fluid: to a lesser or greater degree the viscosity of the crosslinked fluid is reduced after it has been exposed to high shear (1000/s) which is common when displacing the fluid in a workstring to the perforations. The time for the fluid to recover viscosity after being exposed to high shear may take minutes and it is during this time that the fluid/proppant is entering into the hydraulic fracturing. The reduced viscosity of the fluid results in a narrower hydraulic fracture and so increase the risk of the proppant screening out in the well bore.
To combat these problems associated with polymeric gelling agents, some surfactants have been used as gelling agents. In particular cases, some surfactants, when mixed with an aqueous fluid having a certain ionic strength, are capable of forming a viscous fluid that has certain elastic properties, one of which may be shear thinning. Surfactant molecules (or ions) at specific conditions may form micelles (e.g., worm-shaped micelles, rod-shaped micelles, etc.) in an aqueous fluid. Depending on, among other things, the surfactant concentration, and the ionic strength of the fluid, etc., these micelles may impart increased viscosity to the aqueous fluid, such that the fluid exhibits viscoelastic behavior due, at least in part, to the association of the surfactant molecules contained therein.
As a result, these treatment fluids exhibiting viscoelastic behavior may be used in a variety of subterranean treatments where a viscosified treatment fluid may be useful. Because the micelles may be sensitive to the pH and hydrocarbons, the viscosity of these treatment fluids may be reduced after introduction into the subterranean formation without the need for conventional gel breakers (e.g., oxidizers). This may allow a substantial portion of the treatment fluid to be produced back from the formation without the need for expensive remedial treatments.
In the same way, fracturing fluids with viscoelastic surfactants have also several inherent characteristics. As a solids free fluid, they may not create residual damage in either proppant pack or the faces of the fractures. As a solids free fluid, they may have limited fluid loss control in high permeability formations. No filter cake is formed so the fluid loss may be a function of the viscosity of the fluid, permeability of the formation and properties of the reservoir fluids. One fluid can easily displace the other in the porous medium under reservoir conditions. High concentrations of surfactant arc required to create a fluid with sufficient viscosity to create a hydraulic fracture in any formation with permeability greater than a few millidarcy. The viscosity of a fluid with a given concentration is very sensitive to any change in temperature above 150 Deg F. and in almost every case drops dramatically. Compatibility with formation crude as the VES viscosity is very sensitive to the presence of surfactants or demulsifiers.
The objective is to create a hybrid fluid which combines a low concentration of VES and a crosslinked polymer fluid. The final fluid will overcome to some degree the technical and economic disadvantages of crosslinked polymer and VES fluids taken separately.